1. The Field of the Invention
The invention relates to a method for supporting and transporting hot flat glass on a gas bed, which is built up over a gas-permeable support surface of a base, in a contactless manner by passing a gas through the support surface.
The invention also relates to a device for performing this method.
2. Related Art
In the conventional glass ceramicizing process typically used in the industry, the glass to be ceramicized, typically in the form of a plate-shaped or fiat green glass body, rests on a solid base. Since during the ceramicization process temperatures and viscosities are reached at which the glass surface can be harmed by mechanical contact with the base, spot-shaped impressions due to adhesion known as “pits” appear on the underside of the glass. Due to shrinkage occurring during ceramicization, relative motion between the supported glass and the base also occurs, creating scratches on the glass surface.
These disadvantages, which result from contact of the green glass plate with the solid base, seemingly suggest employing principles from float glass production. However, the ceramicization process differs from the usual float glass tempering process in that substantially higher temperatures are required, usually up to about 950° C., but in special cases even up to 1250° C., instead of about 500° C. to 700° C. in the case of tempering and bending furnaces for float glass. In addition, temperature homogeneity in the range of a few degrees K must be guaranteed to prevent deformations of the glass ceramic.
It is known to ceramicize glass in contactless fashion by retaining the green glass molded body to be ceramicized on a gas bed in which the supporting gas, as a rule air, flows out of a permeable base and builds up a load-bearing air bed between this base and the green glass molded body. Since there is no direct contact between a solid base and the green glass body, the aforementioned disadvantages cannot arise.
British Patent 1,383,202 discloses a corresponding device for contactless ceramicizing of a green glass plate on an air bed. However this known device has various disadvantages, because air is only supplied through perforated plates but is not drawn off through the base. With typical perforations in the plates, a comparatively high permeability is achieved.
Calculations show that in the middle region of the green glass plate to be ceramicized the gas speed is nearly zero and a “static” pressure is generated. Then a gas flow from the gas bed occurs essentially only in the vicinity of the edge of the plate. In contrast to the pressure in the edge region, the static pressure in the middle of the plate does not change in response to changes in the thickness of the gas film. Accordingly, in the plate center there are no restoring forces responding to deviations from planarity or from a particularly desired shape. Furthermore the thickness of the gas film is very sensitively dependent on the magnitude of the pilot pressure and other process parameters. Disruptions in these latter parameters can therefore cause warping of the plates.
U.S. Pat. No. 3,607,198 describes a device for contactless retention and transport of a hot glass plate on an air cushion that is built up above a solid base. Here, by arranging inlet-air slits and outlet-air slits successively alternating along the transport route zones with static and dynamic gas pressure are created, each extending over the entire width of the glass plate. Since here the gas delivery is effected via narrow slits with large interstices between them, the result is a gas supply that is locally very inhomogeneous, which in turn—because of the alternation between zones of high and low gas speed—makes it difficult to obtain a homogeneous temperature profile in the glass. Moreover, an expensive structure is employed, which is not suitable for use in the range of temperatures around 950° C., and certainly not for higher temperatures of up to about 1250° C. which can be necessary for ceramicization, because of considerations regarding the materials that are used.
U.S. Pat. No. 5,078,775 also discloses a device for contactless support and transport of a flat glass plate, in the tempering of float glass, on an air cushion that is built up above a solid base that likewise has alternating inlet-air and outlet-air slits. This known device, despite modifications in details, relies on the fundamental concept of U.S. Pat. No. 3,607,198, with the inlet-air and outlet-air slits in the base, and thus with regard to ceramicization applications has comparable disadvantages.